This application is based on International Application No. PCT/IL98/00353, filed Jul. 30, 1998, hereby incorporated by reference in its entirety, which is based on Israeli Patent Application No. 121,527 filed on Aug. 12, 1997.
The field of the present invention is electrical heating systems, in particular, alternating electrical current heating systems.
One of the known systems for heating liquids comprises a heating body which consists of a filament, wrapped in an electrical-insulation layer such as ceramic, which is further wrapped in a encapsulation layer such as stainless steel. The rate of heating in such a heating body is dependent on the rate of heat transmittance of the above layers, and is limited thereto. One disadvantage of such a system is the complexity of said heating body, which determines its relatively high cost. Another disadvantage is the fact that when scale deposits on said heating body, it acts as a thermal insulating layer and thus decreases the efficiency of heating. Another disadvantage of said system is that the filament may overheat and burn if there is no liquid present in the liquid container at the time of operation, or if a sufficient scale layer deposits on said heating body and causes the filament to be substantially thermally insulated from the liquid.
A known system of heating electrically-conductive liquids, such as water, is by applying electrical current through said liquid. Such a system comprises a current source connected to two electrodes, each at one pole thereof, and said electrodes are immersed in said liquid. Said liquids, having a relatively high electrical resistance, act as a resistor and are heated as a result of electrical current passing through them. The above heating system may be carried out by utilizing both direct current (DC) or alternating current (AC). Such a heating system is described, e.g., in U.S. Pat. No. 4,730,098.
One disadvantage of a system in which direct current is applied is, when scale deposits on the electrodes, it constitutes an electrically insulating layer, which cuts off the electrical circuit. Another disadvantage of such a DC system is that when common metals such as copper, iron and zinc are utilized as electrodes in such systems, metal dissolves into the liquid via anodic dissolution. In many applications, such ions are considered to be a contamination.
An advantage of said AC system in comparison with a DC system is that the dissolution of metal ions into the liquid is substantially lower. However, even in AC systems, dissolution of electrodes occurs at a rate which is not acceptable by many applications.
It is an object of the present invention to provide an AC electrode heating system in which the electrodes show high resistance to dissolution into the heated medium.
It is a further object of the present invention to provide a heating system in which the deposition of scale on the electrodes does not interfere with or decrease the efficiency of heating.
It is a further object of the present invention to provide a heating system which is not damaged if the heated medium is absent or in case of the deposition of scale on the heating body.
It is a further object of the present invention to provide an improved water softening system.
It is a further object of the present invention to provide an improved method of purifying water from organic impurities.
The term xe2x80x9cscalexe2x80x9d as used herein defines a hard incrustation usually rich in calcium and magnesium salts, such as CaCO3, MgCO3 and Mg(OH)2, that is deposited on a heating body during the operation thereof.
A noble metal is a metal that has outstanding resistance to chemical environments and to oxidation even at high temperatures, such as Rhenium, Ruthenium, Rhodium, Palladium, Silver, Iridium, Platinum and Gold.
A passive metal is a metal in a form in which its outer layer has outstanding resistance to reaction with the environment. In case the passive metal is not a noble metal, said outer layer is usually an oxide of said metal. A passive alloy is an alloy in a form in which its outer layer has outstanding resistance to reaction with the environment. Usually, said outer layer is formed by the oxidation of at least one of the alloying metal elements which constitutes the alloy. Said oxide layer of passive metals and passive alloys is not electrically conductive and therefore such a metal cannot be utilized as an electrode in a DC heating system, as described above.
The present invention relates to a self-protecting heating system which comprises an electrical circuit comprising an alternating current source, at least two electrodes made of passive metals, passive alloys, noble metals or alloys thereof, and electrically-conductive medium wherein two poles of said alternating electrical current source are in electrical contact with at least one electrode at each pole, and said electrodes are both in electrical contact with said electrically-conductive medium.
The present invention further relates to a process of heating which comprises applying alternating electrical current via a circuit as described above.
Said heating process can be applied in numerous processes that require heating such as evaporating, pyrolysis, defrosting, dissolving, chemical reactions, scaling, pasteurizing and sterilizing.
The present invention further relates to passive metals and passive alloys and noble metals and alloys thereof for use as electrodes in an alternating-current heating system, as described above.
Said heating system optionally further comprises an earthing cable which is in contact with the above-mentioned electrically-conductive medium, or with any other matter which is in direct or indirect physical contact with said alternating current source.
Said electrically-conductive medium is optionally a liquid, preferably an aqueous solution or water.
Said passive metals are preferably chosen from a group consisting of Aluminum, Titanium, Cobalt, Nickel, Niobium, Tantalum, Zirconium, Molybdenum, Chromium, Hafnium, and Tungsten.
Said passive alloys are preferably feritic stainless steels such as AISI 420, AISI 430, austenitic stainless steels such as AISI 304, AISI 316, AISI 321, AISI 347, AISI 904, duplex stainless steels such as ferallium 255, titanium alloys such as grade 1 through grade 12, and aeronautic alloys thereof, cobalt base super alloys, nickel base super alloys, niobium alloys, tantalum alloys and tungsten alloys.
Said alloys of noble metals are preferably Gold-Silver-Copper alloys, Gold-Silver-Platinum alloys, Gold-Copper alloys, Gold-Platinum alloys, Gold-Palladium alloys, Palladium-Platinum alloys and Palladium-Copper alloys.
Said heating system optionally further comprises a thermostat which disconnects the electrical power from the electrodes at a desirable temperature such as the boiling temperature of the heated medium.